Remove unnecessary `object` base class in docs (#10366).

Also add a note about inheritance from `object` being default.
diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index 349df00..63b25bb 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -1312,7 +1312,7 @@
 that is normally used.  This can be achieved by specifying the ``namespace=``
 keyword argument::
 
-   >>> class C(object):
+   >>> class C:
    ...     pass
    ...
    >>> c = C()